Hemoglobin A1c (Hb1c) is the gold standard for assessing diabetes control yet some patients have HbA1c discordant from plasma glucose. We developed a measure of the discordance between Hb1c and a plasma test of glycemic control, the "Glycosylation Gap" (GG). The GG is highly reproducible, correlates with an important clinical outcome, diabetic nephropathy, and appears genetically linked. A parallel measure of variance between HbA1c and glucose monitoring (Hb glycosylation index), has been shown to predict retinopathy and nephropathy in the DCCT. These data suggest that variation in HbA1c, expressed as the GG, arises from biological mechanism(s) that contribute to diabetic complications in here-to-fore unrecognized ways. Since HbA1c is determined by red blood cell (RBC) glucose concentration, the rate of Hb glycosylation and RBC lifespan, this proposal examines the variability of these factors in diabetic and non-diabetic populations. We found substantial variation in RBC glucose relative to extracellular glucose. This in vitro erythrocyte membrane glucose gradient (EMGG) correlated with GG and by inference with a higher risk of complications. The fact that the RBC and the endothelial cell - the major cell type in which diabetic complications occur - share the same glucose transporter, GLUT1, raises the question whether this finding is mediated by shared GLUT1 variation in the two tissues. These studies will seek to determine: (1) whether variation in GLUT1 function contributes to variation in HbA1c; (2) how variation in RBC lifespan or HbA1c formation rate contributes to HbA1c variability; (3) how long-term glycemic control affects the EMGG and the GG, and whether they represent reproducible phenotypes for genetic studies. These studies provide a comprehensive approach to understanding important sources of variation in HbA1c. The variation in EMGG could account for 11% of total variation in >30 million HbA1c measurements yearly in the United States. This proposal extends the consideration of complications risk from the glucose concentration in the plasma to the glucose concentration inside the cell. The proposal therefore has potentially important basic science, clinical and population implications.